A sensor 11 detects the acoustic pressure at a first location in a combustion chamber 7 of a test rig 1 and produces an input signal which is a function of the acoustic pressure. A controller 13 receives the input signal and produces an output signal which is a function of the input signal. An acoustic actuator (16-18) receives the output signal and introduces into the combustion chamber 7 at a second location an acoustic pressure which is a function of the output signal. The acoustic actuator may comprise a fuel injector 18 or a loudspeaker. By using an appropriate control algorithm, acoustic boundary conditions corresponding to a particular gas turbine combustion chamber may be produced, at least in a certain frequency range.